Mechanical and chemical-mechanical planarizing processes (collectively “CMP processes”) remove material from the surface of semiconductor wafers, field emission displays, or other microfeature workpieces in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a carrier assembly 30, and a planarizing pad 40. The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow F), or it reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier assembly 30 has a head 32 to which a microfeature workpiece 12 may be attached, or the microfeature workpiece 12 may be attached to a resilient pad 34 in the head 32. The head 32 may be a free-floating wafer carrier, or an actuator assembly 36 may be coupled to the head 32 to impart axial and/or rotational motion to the workpiece 12 (indicated by arrows H and 1, respectively).
The planarizing pad 40 and a planarizing solution 44 on the pad 40 collectively define a planarizing medium that mechanically and/or chemically removes material from the surface of the workpiece 12. The planarizing pad 40 can be a soft pad or a hard pad. The planarizing pad 40 can also be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution 44 is typically a non-abrasive “clean solution” without abrasive particles. In other applications, the planarizing pad 40 can be a non-abrasive pad composed of a polymeric material (e.g., polyurethane), resin, felt, or other suitable materials. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically abrasive slurries with abrasive particles suspended in a liquid. The planarizing solution may be replenished from a planarizing solution supply 46.
In chemical-mechanical planarization (as opposed to solely mechanical planarization), the planarizing solution 44 will typically chemically interact with the surface of the workpiece 12 to control the removal rate or otherwise optimize the removal of material from the surface of the workpiece. Increasingly, microfeature device circuitry (i.e., trenches, vias, and the like) is being formed from copper. When planarizing a copper layer using a CMP process, the planarizing solution 44 is typically neutral to acidic and includes an oxidizer (e.g., hydrogen peroxide) to oxidize the copper and increase the copper removal rate. One particular slurry useful for polishing a copper layer is disclosed in International Publication Number WO 02/18099, the entirety of which is incorporated herein by reference.
To planarize the workpiece 12 with the CMP machine 10, the carrier assembly 30 presses the workpiece 12 face-downward against the planarizing medium. More specifically, the carrier assembly 30 generally presses the workpiece 12 against the planarizing solution 44 on a planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 move to rub the workpiece 12 against the planarizing surface 42. As the workpiece 12 rubs against the planarizing surface 42, material is removed from the face of the workpiece 12. In some common CMP machines 10, the pressure of the workpiece 12 against the planarizing medium may be gradually ramped up and/or ramped down over a period of time instead of immediately pressing the workpiece against the planarizing medium with full force and immediately terminating pressure when the planarizing step is complete.
CMP processes should consistently and accurately produce a uniformly planar surface on the workpiece to enable precise fabrication of circuits and photo-patterns. During the construction of transistors, contacts, interconnects and other features, many workpieces develop large “step heights” that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to meet tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microfeature devices on a workpiece.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts and damascene lines). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is “under-planarized,” or components on the substrate may be destroyed if it is “over-polished.” Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is determined using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under similar conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate or other variables may change from one substrate to another.
To compensate for changes in planarizing conditions (e.g., degradation of the planarizing pad 40, variations in the composition of the planarizing solution 44, or temperature fluctuations), conventional CMP tools predict the estimated planarizing time for the next workpiece 12 using a calculated material removal rate from the preceding workpiece or several preceding workpieces. Typically, this will involve measuring the thickness of the workpiece in a pre-planarizing metrology tool, planarizing the workpiece on the CMP machine 10, and measuring the thickness of the workpiece again in a post-planarizing metrology tool. Dividing the change in the measured thickness by the time spent planarizing a microfeature workpiece 12 can determine the material removal rate for that particular workpiece. The calculated removal rate may be used as an estimated removal rate for the next workpiece on the assumption that the planarizing conditions will not change too greatly between two sequentially processed workpieces.
To mask statistical variation from one workpiece to another, many CMP machines 10 use an exponentially weighted moving average of material removal rates from a series of microfeature workpieces to predict the material removal rate for the next workpiece. Aspects of such exponentially weighted moving average controllers, among other CMP controllers, are described in some detail in U.S. Pat. No. 6,230,069, the entirety of which is incorporated herein by reference.
Some commercially available CMP machines employ two different types of planarizing pads 40, each mounted on a separate platen 20. A first planarizing pad may remove material at a relatively fast rate and a second planarizing pad may be a finishing pad that removes material at a slower rate to yield a highly polished surface. Applied Materials Corporation of California, USA, sells one such CMP machine under the trade name MIRRA MESA. To increase throughput, the MIRRA MESA CMP tool includes two rough planarizing pads and one finishing pad. The material removal rate for the MIRRA MESA machine is calculated in much the same fashion as other conventional CMP machines, i.e., the total change in thickness as a result of processing on the CMP machine is divided by the combined primary planarizing time on the two rough planarizing pads, which tends to be the only planarizing time that is adjusted from one workpiece to the next.
To estimate the planarizing time necessary to planarize an incoming microfeature workpiece, the thickness of the top layer(s) on the incoming workpiece can be measured to determine the amount of material that needs to be removed. The estimated planarizing time may then be calculated using the formula:
      t          i      ⁢                          ⁢      n        =      t    +                  KE        +                              K                          i              ⁢                                                          ⁢              n                                ⁢          Δ          ⁢                                          ⁢                      T                          i              ⁢                                                          ⁢              n                                      +                  rI          ⁡                      (                          E              ′                        )                              RR      
wherein:                tin is the estimated planarizing time of an incoming workpiece;        t is the actual planarizing time of the preceding workpiece;        K is an empirically determined constant;        E is the difference between the predicted final thickness of the preceding workpiece and the thickness actually measured by the post-planarizing metrology tool;        Kin is another empirically determined constant;        ΔTin is the thickness of the material to be removed from the incoming workpiece;        r is another empirically determined constant;        I(E′) is an integral function (e.g., of the type commonly employed in PID control systems) of the difference between a predicted final thickness and the actually measured thickness for a series of preceding workpieces; and        RR is the calculated removal rate. This calculated removal rate may be the removal rate for the immediately preceding workpiece or may be an average, e.g., an exponentially weighted moving average, of a number of preceding workpieces.        
The estimated planarizing time calculated in such a fashion can be a reasonably accurate estimate if the amount of material to be removed from the workpiece is relatively large, e.g., several thousand angstroms. With advances in the design of workpieces, the layers of material being removed in the CMP process is decreasing over time, with some CMP processes removing less than 1,000 Å. The conventional techniques outlined above for estimating the planarizing time for a given workpiece are proving less accurate at predicting material removal rate as the amount of material being removed is reduced. This greater variability in calculated removal time, together with the reduced amount of material being removed, can lead to materially under-planarizing or over-planarizing the workpieces.